The invention relates to measuring gas levels using an electrochemical cell.
In such measurements, the air containing the gas to be measured is brought into contact with the cell, which contains at least an anode and a cathode and preferably also a reference electrode in contact with an electrolyte. The gas diffuses through the thin layer of electrolyte covering the anode and adsorbs onto the anode as the potential applied between the anode and cathode or reference electode is maintained at a constant level. Typically, the steady state current resulting from the reaction of the gas at the anode surface is measured to determined the amount of the gas present.
LaConti and Maget, J. Electrochem. Soc., Vol. 118, p. 506 (1971), discloses a platinum gas diffusion (fuel cell type) electrode (anode) sprayed onto the surface of a solid polymer electrolyte, including sulfonated fluorinated hydrocarbon as a carbon monoxide (CO) detector. The lower limit of the operating range of the electrode is 0.71V vs. RHE (reversible hydrogen electrode).
Blurton and Sedlack, J. Electrochem. Soc., Vol. 121, p. 1315 (1974), discloses a platinum gas diffusion electrode in sulfuric acid, operating in a range of 0.9V to 1.5V vs. RHE, measuring steady state CO oxidation currents and using an internal reference electrode.
Oswin, U.S. Pat. No. 3,776,832, describes an electrochemical cell used for detection of noxious gases which includes an anode, a cathode, and a reference electrode in an aqueous electrolyte. A fixed potential is maintained between the anode and the reference electrode; the gas reacts at the anode and the resulting current is measured to determine the amount of the gas present in the sample. For CO measurements, the anode, cathode and reference electrodes are platinum/Teflon diffusion electrodes, and sulfuric acid is used as the electrolyte. The cell operates within the range of 0.9V to 1.5V vs. RHE.
The methods of LaConti et al., Blurton et al., and Oswin all rely on restricting the potential range of their potentiostatic electrodes to potentials where the current due to oxygen reduction and to oxygen evolution (oxygen interference) is negligible.
Stripping voltammetry is an alternative method, and a more sensitive one, for measuring the concentration of an electroactive species in a gas in contact with an electrode. Gilman, J. Phys. Chem., Vol. 67, p. 78 (1963) demonstrated this technique for measuring CO in an inert gas. The technique consists of allowing the active component, for example CO, to adsorb on the electrode at a potential where it does not oxidize and then anodically sweeping the potential and measuring the current resulting from oxidation of the adsorbed gas.
Although stripping voltammetry has many advantages over the steady state method, it could not be applied to the quantitative measurement of CO or other electroactive gaseous species in air because of interference by reduction of oxygen at the adsorbing potentials. For example, Breiter, Proc. Symp. on Electro-catalysis, Vol. 82-2, p. 102 (1982), has shown that reduction of oxygen removes CO adsorbed on a platinum electrode. The present invention discloses formulations of electrolytes which permit the use of stripping voltammetry even when oxygen is present in the gaseous mixture.